Does anyone know why Ford specifies 5W-50 viscosity? If the 5W spec is for winter use, and those who don't drive (or more importantly, start up) in really cold temps, we could substitute 15W-50, which is commonly available and much less $$. On the high end of the viscosity spec, the highest lubrication stress in the engine is the cam/follower interface (this is the primary need for ZDDP in conventional oils). But the cams in GT engines are not aggressive profiles (don't need to be with forced induction to get more air into the engine), and equivalent engines in the F-series trucks don't specify that high of viscosity. Furthermore, the Ford "modular engines" use roller followers, which greatly reduce the stress on the cam/follower interface (including our GT engines). In fact, Ford specifies 5W-20 for most engines produced since the early 2000's, including the other 5.4's, and 5W-30 for the Ecoboost engines. Why 50 for the GT engine?
Any experts here have insight into Ford's reasoning? Was it just overkill since they didn't have time for a full test/development cycle? TIA